Hydraulic action

Hydraulic action is a form of mechanical weathering caused by the force of moving water currents rushing into a crack in the rockface. The water compresses the air in the crack, pushing it right to the back. As the wave retreats, the highly pressurised air is suddenly released with explosive force, capable of chipping away the rockface over time. Thus, the crack is gradually widened so the amount of compressed air increases like a balloon, and hence the explosive force of its release increases. Thus, the problem intensifies (a 'positive feedback' system).It helps the river to get lower (rejuvination). It is helped in part on cliffs by 'Freeze Thaw'.